Mohamedou Ould Slahi is a Mauritanian electrical engineer who studied in Germany before immigrating to Canada, where he served briefly as imam at the Al-Sunnah mosque in Montreal.

He is also Canada’s alleged link to 9/11.

According to the U.S. military, Mr. Slahi is a key al-Qaeda member who recruited three of the hijackers who carried out the Sept. 11, 2001 terrorist attacks.

Since 2002, he has been held at Guantanamo Bay. The latest batch of documents released by WikiLeaks explains why: the U.S. thinks he is a “high risk” and still holds secrets about al-Qaeda, including its activities in Canada.

The classified documents disclosed by WikiLeaks are assessments of almost 800 past and present Guantanamo detainees. On a scale of low, medium or high, they rate the detainees for their intelligence value and the risks they could pose if released.

The assessment of Mr. Slahi calls him “one of the most valuable sources” at Guantanamo, but says while he has been highly co-operative he has more to tell, such as details on “members, facilitators, and financiers within al-Qaeda’s Canadian cells.”

“Detainee still has useful information regarding extremist activity in North Africa, Europe, and Canada, as well as information concerning the 11 September 2001 terrorist attacks,” reads the March 3, 2008 assessment.

The leaked Department of Defense documents were given to selected news outlets, and some were posted on the Internet. While they spell out U.S. concerns about detainees, in some cases they appear short on evidence, often relying on sources of unknown credibility.

In the case of Mr. Slahi, a U.S. District Court ruled last year the government had failed to establish he was part of al-Qaeda, only that “from time to time, before his capture, he provided sporadic support to members of al-Qaeda.”

“The government’s problem is that its proof that Slahi gave material support to terrorists is so attenuated, or so tainted by coercion and mistreatment, or so classified, that it cannot support a successful criminal prosecution,” Judge James Robertson wrote. The judge ordered his release but an appeals court vacated the decision.

The 12-page assessment of Mr. Slahi describes him as “an admitted member of al-Qaeda” who swore an oath to Osama bin Laden and “was prepared to be a martyr.” It says he had “probably” served as lieutenant to his cousin, Abu Hafs al-Mauritani, bin Laden’s religious advisor and a member of the al-Qaeda council.

“Detainee is assessed to be the leader of the Montreal-based al-Qaeda cell responsible for the foiled ‘Millennium bomber’ plot targeting Los Angeles International Airport and possibly other U.S.-based targets in December 1999,” it adds.

Mr. Slahi, 40, left Mauritania as a teenager to study in Duisburg, Germany, on a grant by a foundation named after the German industrialist Carl Duisburg. While at the University of Duisburg, he “received extremist religious indoctrination,” the assessment says.

He travelled to Afghanistan “for training and jihad” in 1991, spending six weeks at the Al Farooq camp near Kandahar. In 1992, he went back to Afghanistan to fight the Soviet-backed Afghan government.

Next, he tried to “join the jihad” in Bosnia but returned to Germany after a week because it was too dangerous.

“He admitted his primary responsibility was to recruit for al-Qaeda in Europe,” says the assessment.

Among those he allegedly recruited were Muhammad Atta, Marwan al-Shehhi and Ziad Jarrah, who would later pilot three of the planes during the 9/11 attacks.

By 1999, Mr. Slahi was having troubles extending his German visa so he moved to Canada, which had accepted him as a landed immigrant. He arrived in Montreal on Nov. 26, 1999 and stayed at a safe house used by members of a local terrorist cell.

Soon after his arrival, he met Ahmed Ressam, the Algerian terrorist, who was then preparing to bomb Los Angeles airport. After Ressam was arrested while trying to cross the U.S. border with the bomb hidden in his car, the RCMP questioned Mr. Slahi.

He left Canada soon after that. Arrested in 2001, he was transferred to Jordan, then to the U.S. military base in Bagram, Afghanistan. On Aug. 5, 2002, he arrived at Guantanamo Bay.

The U.S. assessment says “multiple independent sources” have corroborated his involvement in 9/11 recruitment, Ressam’s bomb plot and the April 11, 2002, truck bombing of a synagogue in Tunisia. Some of those sources are named in the report: al-Qaeda operative Ramzi Bin al-Shibh and a former Montrealer named Ahmed Laabidi.

But while Mr. Slahi has given U.S. authorities “valuable intelligence,” such as information about how al-Qaeda had invested in “unwitting companies” in Canada and elsewhere, he has denied any direct involvement in terrorism.

“He is highly intelligent and has been cooperative, as long as it does not link him to terrorist activities beyond what he has already conceded: that he is an admitted member of al-Qaeda with family ties to a senior member of the organization.”